In the art of window assemblies which include rectangular window and sash frames formed of sections of extruded rigid vinyl or similar plastics material, it is desirable to have rigid fusion-welded mitered corner joints on all of the frames while minimizing the volume of plastics material in each section. It is also desirable for the window frame to have vertical jamb members with a different cross-sectional configuration or profile than that of the horizontal head member or that of the sill member of the frame and to minimize the height of the sill member. For example, it is desirable for the sill member of a double-hung window frame to have an outwardly and downwardly sloping wall while the head member defines a channel for receiving the upper sash frame. On the other hand, it is desirable for each of the vertical jamb members of the window frame to define a pair of channel-like tracks for receiving support slides or shoes which pivotally support the sash frames. Preferably, the jamb members also define smaller channels for receiving and retaining a rectangular screen frame. However, the different cross-sectional profiles for the jamb, head and sill members make it difficult to obtain rigid and clean looking fusion-welded mitered corner joints while also minimizing the volume of plastics material in each member.